Low-molecular-ratio cryolite for aluminium electrolytic industry and method for preparing the same

ABSTRACT

The disclosure provides low-molecular-ratio cryolite for aluminum electrolytic industry, which consists of potassium cryolite and sodium cryolite with a mole ratio of 1:1˜1:3, wherein the molecular formula of the potassium cryolite is mKF.AlF 3  and the molecular formula of the sodium cryolite is nNaF.AlF 3 , where m=1˜1.5 and n=1˜1.5. When the low-molecular-ratio cryolite provided by the disclosure is applied to the aluminum electrolytic industry, electrolytic temperature and power consumption can be reduced and electrolytic efficiency is improved.

TECHNICAL FIELD OF THE INVENTION

The disclosure relates to cryolite, and in particular to thelow-molecular-ratio cryolite for aluminium electrolytic industry and amethod for preparing the same.

BACKGROUND OF THE INVENTION

At present, aluminium electrolytic industry still employs a conventionalHall-Heroult process; electrolyte always takes cryolite-aluminium oxideas a basic system, and the cryolite generally adopts sodiumfluoroaluminate. The aluminium electrolytic industry needs anelectrolytic temperature of about 960 DEG C. and thus power consumptionis high, this is mainly because the liquidus temperature of theelectrolyte is high and it is necessary to keep a certain temperature ofsuperheat degree to make the aluminium oxide have a better solubility.

The method for preparing cryolite in industry generally adopts asynthesis method, in which anhydrous hydrofluoric acid reacts withaluminium hydroxide to form fluoaluminic acid; then the fluoaluminicacid reacts with sodium hydroxide or potassium hydroxide at a hightemperature; after processes of filtering, drying, melting and crushing,the cryolite is prepared, wherein the cryolite synthesized by thismethod has a molecular ratio of m=3.0, with a relatively high meltingpoint. The cryolite synthesized by the existing industrial synthesismethod has a molecular ratio of m=2.0-3.0, and it is difficult to obtainthe relatively pure low-molecular-ratio cryolite containing extremelylow water content with a molecular ratio of m=1.0-1.5.

Therefore, the conventional art has disadvantages that the electrolyticpower consumption is high and the electrolyte is not ideal.

SUMMARY OF THE INVENTION

In order to solve the technical problem existing in the conventionalart, the inventor has done a great deal of research in the selection andpreparation of electrolyte and unexpectedly finds that taking themixture of low-molecular-ratio potassium cryolite andlow-molecular-ratio sodium cryolite with a certain ratio as theelectrolyte of the aluminium electrolytic system can significantlyreduce the electrolytic temperature compared with the conventionalaluminium electrolytic system which takes sodium cryolite as theelectrolyte, and has obvious advantages in corrosion to electrodematerials compared with the aluminium electrolytic system which takessingle low-molecular-ratio potassium cryolite or low-molecular-ratiosodium cryolite as the electrolyte, but has the electrolytic temperaturedecrease fallen in between the conventional aluminium electrolyticsystem which takes sodium cryolite as the electrolyte and the aluminiumelectrolytic system which takes single low-molecular-ratio potassiumcryolite or low-molecular-ratio sodium cryolite as the electrolyte.

The disclosure provides low-molecular-ratio cryolite for aluminiumelectrolytic industry, which consists of potassium cryolite and sodiumcryolite with a mole ratio of 1:1˜1:3, wherein the molecular formula ofthe potassium cryolite is mKF.AlF₃, m=1˜1.5; the molecular formula ofthe sodium cryolite is nNaF.AlF₃, n=1˜1.5.

With the technical scheme above, when the low-molecular-ratio cryoliteprovided by the disclosure is applied to the aluminium electrolyticindustry, the solubility property of aluminium oxide is improved, thus,the electrolytic temperature is reduced, the power consumption isreduced and the electrolytic efficiency is improved.

As a further improvement of the disclosure, m=1, 1.2 or 1.5; whenm=1.0˜1.5; the melting point of the potassium cryolite mKF.AlF₃ isbetween 540 and 570 DEG C., wherein the melting point of the mKF.AlF₃slightly increases as the increase of m. n=1, 1.2 or 1.5; whenn=1.0˜1.5, the melting point of the nNaF.AlF₃ is between 960 and 1000DEG C., wherein the melting point of the sodium cryolite nNaF.AlF₃slightly increases as the increase of n.

As a further improvement of the disclosure, the mole ratio of thepotassium cryolite to the sodium cryolite is 1:1; m=1.5 and n=1.5; thealuminium oxide has a solubility of 13 g/l in the system consisting ofpotassium cryolite and sodium cryolite with a mole ratio of 1:1 and theelectrolytic temperature is between 825 and 900 DEG C.

Correspondingly, the disclosure also provides a method for preparing thelow-molecular-ratio cryolite for aluminium electrolytic industry, whichincludes the following steps:

A) putting aluminium into a reactor, injecting an inert gas to thereactor after vacuumizing, heating the reactor to a temperature ofbetween 700 and 850 DEG C., adding potassium fluotitanate, potassiumfluoborate or mixture of they two in the reactor and stirring for 4 to 6hours, pumping the superstratum melt liquid to obtain potassiumcryolite; putting aluminium into another reactor, injecting an inert gasto the reactor after vacuumizing, heating the reactor to a temperatureof between 700 and 850 DEG C., adding sodium fluotitanate, sodiumfluoborate or mixture of they two in the reactor and stirring for 4 to 6hours, pumping the superstratum melt liquid to obtain sodium cryolite;and

B) mixing the obtained potassium cryolite with the obtained sodiumcryolite in a mole ratio of 1:1˜1:3.

The preparation method provided by the disclosure has advantages of mildreaction conditions, easy control, simple process, full reaction andhigh quality of reaction product.

As a further improvement of the disclosure, the method for preparing thelow-molecular-ratio cryolite for aluminium electrolytic industryincludes the following steps:

A) putting aluminium into a reactor, injecting an inert gas to thereactor after vacuumizing, heating the reactor to a temperature ofbetween 780 and 850 DEG C., adding potassium fluotitanate in the reactorand stirring for 4 to 6 hours, pumping the superstratum melt liquid toobtain potassium cryolite of which the molecular formula is

${\frac{3}{2}{{KF} \cdot {AlF}_{3}}};$putting aluminium into another reactor, injecting an inert gas to thereactor after vacuumizing, heating the reactor to a temperature ofbetween 780 and 850 DEG C., adding sodium fluotitanate in the reactorand stirring for 4 to 6 hours, pumping the superstratum melt liquid toobtain sodium cryolite of which the molecular formula is

${\frac{3}{2}{{NaF} \cdot {AlF}_{3}}};$and

B) mixing the obtained potassium cryolite with the obtained sodiumcryolite in a mole ratio of 1:1˜1:3, wherein the reaction formulainvolved is:

${{{\frac{3}{4}K_{2}{TiF}_{6}} + {Al}} = {{\frac{3}{4}{Ti}} + {\frac{3}{2}{{KF} \cdot {AlF}_{3}}}}};$${{\frac{3}{4}{Na}_{2}{TiF}_{6}} + {Al}} = {{\frac{3}{4}{Ti}} + {\frac{3}{2}{{NaF} \cdot {{AlF}_{3}.}}}}$

As a further improvement of the disclosure, the method for preparing thelow-molecular-ratio cryolite for aluminium electrolytic industryincludes the following steps:

A) putting aluminium into a reactor, injecting an inert gas to thereactor after vacuumizing, heating the reactor to a temperature ofbetween 700 and 850 DEG C., adding potassium fluoborate in the reactorand stirring for 4 to 6 hours, pumping the superstratum melt liquid toobtain potassium cryolite of which the molecular formula is KF.AlF₃;putting aluminium into another reactor, injecting an inert gas to thereactor after vacuumizing, heating the reactor to a temperature ofbetween 700 and 850 DEG C., adding sodium fluoborate in the reactor andstirring for 4 to 6 hours, pumping the superstratum melt liquid toobtain sodium cryolite of which the molecular formula is NaF.AlF₃; and

B) mixing the obtained potassium cryolite with the obtained sodiumcryolite in a mole ratio of 1:1˜1:3, wherein the reaction formulainvolved is:KBF₄+Al=B+KF.AlF₃; NaBF₄+Al=B+NaF.AlF₃.

As a further improvement of the disclosure, the method for preparing thelow-molecular-ratio cryolite for aluminium electrolytic industryincludes the following steps:

A) putting aluminium into a reactor, injecting an inert gas to thereactor after vacuumizing, heating the reactor to a temperature ofbetween 700 and 850 DEG C., adding the mixture of potassium fluoborateand potassium fluotitanate with a mole ratio of 2:1 in the reactor andstirring for 4 to 6 hours, pumping the superstratum melt liquid toobtain potassium cryolite of which the molecular formula is

${\frac{6}{5}{{KF} \cdot {AlF}_{3}}};$putting aluminium into another reactor, injecting an inert gas to thereactor after vacuumizing, heating the reactor to a temperature ofbetween 700 and 850 DEG C., adding the mixture of sodium fluoborate andsodium fluotitanate with a mole ratio of 2:1 in the reactor and stirringfor 4 to 6 hours, pumping the superstratum melt liquid to obtain sodiumcryolite of which the molecular formula is

${\frac{6}{5}{{NaF} \cdot {AlF}_{3}}};$and

B) mixing the obtained potassium cryolite with the obtained sodiumcryolite in a mole ratio of 1:1˜1:3, wherein the reaction formulainvolved is:

${{{K_{2}{TiF}_{6}} + {2{KBF}_{4}} + {\frac{10}{3}{Al}}} = {{TiB}_{2} + {\frac{10}{3}\left\lbrack {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right\rbrack}}};$${{{Na}_{2}{TiF}_{6}} + {2{NaBF}_{4}} + {\frac{10}{3}{Al}}} = {{TiB}_{2} + {{\frac{10}{3}\left\lbrack {\frac{6}{5}{{NaF} \cdot {AlF}_{3}}} \right\rbrack}.}}$

As a further improvement of the disclosure, the method for preparing thelow-molecular-ratio cryolite for aluminium electrolytic industryincludes the following steps:

A) putting excessive aluminium into a reactor, heating the reactor to atemperature of between 700 and 850 DEG C., adding the mixture ofpotassium fluoborate and potassium fluotitanate with a mole ratio of y:xin the reactor and stirring for 0.5 to 6 hours, pumping the superstratummelt liquid to obtain potassium cryolite of which the molecular formulais

${\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {AlF}_{3}}};$putting excessive aluminium into another reactor, heating the reactor toa temperature of between 700 and 850 DEG C., adding the mixture ofsodium fluoborate and sodium fluotitanate with a mole ratio of y:x inthe reactor and stirring for 0.5 to 6 hours, pumping the superstratummelt liquid to obtain sodium cryolite of which the molecular formula is

${\frac{{3y} + {6x}}{{3y} + {4x}}{{NaF} \cdot {AlF}_{3}}};$and

B) mixing the obtained potassium cryolite with the obtained sodiumcryolite in a mole ratio of 1:1˜1:3, wherein the reaction formulainvolved is:

${{K_{2}{TiF}_{6}} + {KBF}_{4} + {\left. {Al}\longrightarrow{Al} \right. \cdot {Ti} \cdot B} + {\frac{{3y} + {6x}}{{3y} + {4x}}{{KF} \cdot {AlF}_{3}}}};$${{Na}_{2}{TiF}_{6}} + {NaBF}_{4} + {\left. {Al}\longrightarrow{Al} \right. \cdot {Ti} \cdot B} + {\frac{{3y} + {6x}}{{3y} + {4x}}{{NaF} \cdot {{AlF}_{3}.}}}$

Compared with the conventional art, the disclosure achieves advantagesas follows: when the low-molecular-ratio cryolite provided by thedisclosure is applied to the aluminium electrolytic industry, thesolubility property of aluminium oxide is improved, thus, theelectrolytic temperature is reduced; and compared with the aluminiumelectrolytic system which takes the conventional cryolite or singlelow-molecular-ratio potassium cryolite or single low-molecular-ratiosodium cryolite as the electrolyte, the electrolytic temperature isobviously different and the corrosion to electrode materials isdifferent too; the method provided by the disclosure for preparing thelow-molecular-ratio cryolite has advantages of mild reaction conditions,easy control, simple process and full reaction.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The disclosure is described below in further detail through specificembodiments.

Embodiment 1

Weighing 1 ton of aluminium and putting it into a reactor, injectingargon to the reactor for protection after vacuumizing, heating thereactor to a temperature of 800 DEG C., adding dried potassiumfluotitanate in the reactor slowly in accordance with a reaction ratioand stirring quickly for 5 hours to form titanium sponge and potassiumcryolite

$\left( {\frac{3}{2}{{KF} \cdot {AlF}_{3}}} \right),$opening the cover of the reactor, pumping the superstratum melt liquidpotassium cryolite through a siphon-pump. Weighing 1 ton of aluminiumand putting it into another reactor, injecting argon to the reactor forprotection after vacuumizing, heating the reactor to a temperature of800 DEG C., adding dried sodium fluotitanate in the reactor slowly inaccordance with a reaction ratio and stirring quickly for 5 hours toform titanium sponge and sodium cryolite

$\left( {\frac{3}{2}{{NaF} \cdot {AlF}_{3}}} \right),$opening the cover of the reactor, pumping the superstratum melt liquidsodium cryolite through a siphon-pump.

Mixing the prepared potassium cryolite

$\left( {\frac{3}{2}{{KF} \cdot {AlF}_{3}}} \right),$with the prepared sodium cryolite

$\left( {\frac{3}{2}{{NaF} \cdot {AlF}_{3}}} \right),$in a mole ratio of 1:1 and applying the cryolite mixture to thealuminium electrolytic industry, wherein the electrolytic temperaturecan be controlled in a range of between 850 and 900 DEG C., and virginaluminium can be obtained by using inert electrode materials or carbonelectrode materials or mixed (combination of carbon and inert electrodematerials) electrode materials to carry out electrolysis.

Embodiment 2

Weighing 1 ton of aluminium and putting it into a reactor, injectingargon to the reactor for protection after vacuumizing, heating thereactor to a temperature of 780 DEG C., adding dried potassiumfluoborate in the reactor slowly in accordance with a reaction ratio andstirring quickly for 5 hours to form boron and potassium cryolite(KF.AlF₃), opening the cover of the reactor, pumping the superstratummelt liquid potassium cryolite through a siphon-pump. Weighing 1 ton ofaluminium and putting it into another reactor, injecting argon to thereactor for protection after vacuumizing, heating the reactor to atemperature of 780 DEG C., adding dried sodium fluoborate in the reactorslowly in accordance with a reaction ratio and stirring quickly for 5hours to form boron and sodium cryolite (NaF.AlF₃), opening the cover ofthe reactor, pumping the superstratum melt liquid sodium cryolitethrough a siphon-pump.

Mixing the prepared potassium cryolite (KF.AlF₃) with the preparedsodium cryolite (NaF.AlF₃) in a mole ratio of 1:1 and applying thecryolite mixture to the aluminium electrolytic industry, wherein theelectrolytic temperature can be controlled in a range of between 825 and900 DEG C., and virgin aluminium can be obtained by using inertelectrode materials or carbon electrode materials or mixed (combinationof carbon and inert electrode materials) electrode materials to carryout electrolysis.

Embodiment 3

Weighing 1 ton of aluminium and putting it into a reactor, injectingargon to the reactor for protection after vacuumizing, heating thereactor to a temperature of 750 DEG C., adding the mixture of driedpotassium fluoborate and potassium fluotitanate in the reactor slowly inaccordance with a reaction ratio, wherein the mole ratio of thepotassium fluoborate to the potassium fluotitanate is 2:1; stirringquickly for 5 hours to form titanium boride and potassium cryolite

$\left( {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right),$opening the cover of the reactor, pumping the superstratum melt liquidpotassium cryolite through a siphon-pump. Weighing 1 ton of aluminiumand putting it into a reactor, injecting argon to the reactor forprotection after vacuumizing, heating the reactor to a temperature of750 DEG C., adding the mixture of dried sodium fluoborate and sodiumfluotitanate in the reactor slowly in accordance with a reaction ratio,wherein the mole ratio of the sodium fluoborate to the sodiumfluotitanate is 2:1; stirring quickly for 5 hours to form titaniumboride and sodium cryolite

$\left( {\frac{6}{4}{{NaF} \cdot {AlF}_{3}}} \right),$opening the cover of the reactor, pumping the superstratum melt liquidsodium cryolite through a siphon-pump.

Mixing the prepared potassium cryolite

$\left( {\frac{6}{5}{{NaF} \cdot {AlF}_{3}}} \right)$with the prepared sodium cryolite

$\left( {\frac{6}{5}{{NaF} \cdot {AlF}_{3}}} \right)$in a mole ratio of 1:1 and applying the cryolite mixture to thealuminium electrolytic industry, wherein the electrolytic temperaturecan be controlled in a range of between 825 and 900 DEG C., and virginaluminium can be obtained by using inert electrode materials or carbonelectrode materials or mixed (combination of carbon and inert electrodematerials) electrode materials to carry out electrolysis.

Embodiment 4

Mixing the prepared potassium cryolite (KF.AlF₃) with the preparedsodium cryolite

$\left( {\frac{6}{5}{{NaF} \cdot {AlF}_{3}}} \right)$in a mole ratio of 1:3 and applying the cryolite mixture to thealuminium electrolytic industry, wherein the electrolytic temperaturecan be controlled in a range of between 850 and 900 DEG C., and virginaluminium can be obtained by using inert electrode materials or carbonelectrode materials or mixed (combination of carbon and inert electrodematerials) electrode materials to carry out electrolysis.

Embodiment 5

Mixing the prepared potassium cryolite

$\left( {\frac{3}{2}{{KF} \cdot {AlF}_{3}}} \right)$with the prepared sodium cryolite (NaF.AlF₃) in a mole ratio of 1:3 andapplying the cryolite mixture to the aluminium electrolytic industry,wherein the electrolytic temperature can be controlled in a range ofbetween 850 and 900 DEG C., and virgin aluminium can be obtained byusing inert electrode materials or carbon electrode materials or mixed(combination of carbon and inert electrode materials) electrodematerials to carry out electrolysis.

Embodiment 6

Mixing the prepared potassium cryolite

$\left( {\frac{6}{5}{{KF} \cdot {AlF}_{3}}} \right)$with the prepared sodium cryolite

$\left( {\frac{3}{2}{{NaF} \cdot {AlF}_{3}}} \right)$in a mole ratio of 1:3 and applying the cryolite mixture to thealuminium electrolytic industry, wherein the electrolytic temperaturecan be controlled in a range of between 850 and 900 DEG C., and virginaluminium can be obtained by using inert electrode materials or carbonelectrode materials or mixed (combination of carbon and inert electrodematerials) electrode materials to carry out electrolysis.

Embodiment 7

Weighing 5 tons of aluminium and putting it into a reactor, heating thereactor to a temperature of 750 DEG C., adding 2 tons of mixture ofdried potassium fluoborate and potassium fluotitanate in the reactorslowly, wherein the mole ratio of the potassium fluoborate to thepotassium fluotitanate is 1:1; stirring quickly for 4 hours to formaluminium-titanium-boron alloy and potassium cryolite

$\left( {\frac{9}{7}{{KF} \cdot {AlF}_{3}}} \right)$due to excessive aluminium, opening the cover of the reactor, pumpingthe superstratum melt liquid potassium cryolite through a siphon-pump.Weighing 5 tons of aluminium and putting it into a reactor, heating thereactor to a temperature of 750 DEG C., adding 2 tons of mixture ofdried sodium fluoborate and sodium fluotitanate in the reactor slowly,wherein the mole ratio of the sodium fluoborate to the sodiumfluotitanate is 1:1; stirring quickly for 4 hours to formaluminium-titanium-boron alloy and sodium cryolite

$\left( {\frac{9}{7}{{NaF} \cdot {AlF}_{3}}} \right)$due to excessive aluminium, opening the cover of the reactor, pumpingthe superstratum melt liquid sodium cryolite through a siphon-pump.

Mixing the prepared potassium cryolite

$\left( {\frac{9}{7}{{KF} \cdot {AlF}_{3}}} \right)$with the prepared sodium cryolite

$\left( {\frac{9}{7}{{NaF} \cdot {AlF}_{3}}} \right)$in a mole ratio of 1:3 and applying the cryolite mixture to thealuminium electrolytic industry, wherein the electrolytic temperaturecan be controlled in a range of between 850 and 900 DEG C., and virginaluminium can be obtained by using inert electrode materials or carbonelectrode materials or mixed (combination of carbon and inert electrodematerials) electrode materials to carry out electrolysis.

The above are the further detailed description of the disclosure made inconjunction with specific preferred embodiments; it can not beconsidered that the specific embodiment of the disclosure is onlylimited to the description above. For the common technicians in thetechnical field of the disclosure, empty simple deductions orsubstitutes can be made without departing from the concept of thedisclosure and they are deemed to be included within the scope ofprotection of the disclosure.

What is claimed is:
 1. Low-molecular-ratio cryolite for aluminiumelectrolytic industry, which consists essentially of a mixture ofpotassium cryolite and sodium cryolite with a mole ratio of 1:1 to 1:3,wherein the molecular formula of the potassium cryolite is mKF.AlF₃,1=m≦1.2; the molecular formula of the sodium cryolite is nNaF.AlF₃,1=n≦1.2.
 2. The low-molecular-ratio cryolite for aluminium electrolyticindustry according to claim 1, wherein m=1 or 1.2; n=1 or 1.2.
 3. Thelow-molecular-ratio cryolite for aluminium electrolytic industryaccording to claim 1, wherein the mole ratio of the potassium cryoliteto the sodium cryolite is 1:1.
 4. The low-molecular-ratio cryolite foraluminium electrolytic industry according to claim 1, wherein the moleratio of the potassium cryolite to the sodium cryolite is 1:3; m=1 andn=1.2.